This invention relates to a data communication device which is to be connected at least to a transmission medium of a data communication network operable in compliance with the CSMA/CD (carrier sense multiple access with collision detection) scheme as called in the art. More particularly, this invention relates to a data communication device for use either as an interface in a transceiver of a CSMA/CD network or as a repeater circuit between a CSMA/CD network and at least one other data communication network which may or may not comprise a CSMA/CD network.
A data communication network operable in accordance with the CSMA/CD scheme, is revealed in U.S. Pat. No. 4,063,220 issued to Robert M. Metcalfe et al, assignors to Xerox Corporation. The transmission medium of the CSMA/Cd network is for transmitting digital signals as data packets. On starting transmission of a data packet, each transceiver of the network senses a carrier transmitted through the transmission medium. The word "carrier" is used in the art to mean another data packet which is transmitted either to the transceiver under consideration or to another transceiver of the network. If no carrier is sensed, the transceiver sends the data packet onto the transmission medium. If a carrier is sensed, the carrier is later sensed again. The transceiver sends the data packet to the transmission medium after having sensed disappearance of the carrier.
If the carrier were not sensed on starting the transmission, a collision would take place on the transmission medium between the data packets by which the transmission medium is simultaneously accessed by two or more transceivers. When the transmission medium is concurrently accessed by two remote transceivers, a collision may take place even with the carrier sensing due to a propagation delay of the data packets along the transmission medium. Each transceiver therefore includes according to Metcalfe et al, a collision detector to detect a collision and produce a collision signal when a collision is occurred. The collision signal is used in suspending transmission of a data packet onto the transmission medium.
Even with the collision detection, two transceivers may concurrently restart the transmission. Another collision is inevitable. A variable interval corresponding to a weighted random number, is therefore preferably used according to Metcalfe et al on restarting the transmission.
Each transceiver preferably includes a packet buffer as an input buffer for temporarily storing a data packet received by that transceiver as a received packet. When a data processing device connected to that transceiver is operable at a low data processing rate, the packet buffer may overflow. Such an overflow results in a loss in the data packet. This unfavorably affects the transmission efficiency of the CSMA/CD network. The data processing device as herein called, is what is named a "using device" by Metcalfe et al.
A repeater circuit is shown in the Metcalfe et al patent in connecting a first segment of the transmission medium with a second segment thereof. The repeater circuit comprises first and second local transceivers for connection to the first and the second segments, respectively, and a repeater logic between the local transceivers.
An improved repeater circuit is disclosed in U.S. Pat. No. 4,099,024 issued to David R. Boggs et al and assigned to Xerox Corporation. In the improved repeater circuit, each local transceiver includes a collision detector of the type used by Metcalfe et al. A collision signal produced in response to a collision on one of the segments, is transmitted to the other segment by the local transceiver connected thereto.
Although not obvious even from the Boggs et al patent, it may be desirable to make the repeater logic include a packet buffer for temporarily storing the data packets received from one of the segments as received packets when a collision takes place between the data packets received from both segments. The packet buffer, however, would give rise to the problems of the type described in connection with the CSMA/CD network according to Metcalfe et al as will later become clear as the description proceeds with reference to several figures of the accompanying drawing.